Research to identify molecules with potential for preventative and therapeutic use (antibodies, enzymes, hormones and vaccine antigens) is of paramount importance to health and medicine. Historically, many of these molecules were recovered from human or animal sources. However, low quantities of target product in the source material coupled with immense costs, and more importantly, safety, have limited the availability of therapeutics and vaccines for prevention and treatment of many diseases around the world.
In the mid-1970s recombinant DNA technology revolutionized the process and made possible the production of target molecules predominantly in bacterial expression systems. Although prokaryotic expression systems continue to be a widely utilized method for recombinant protein production, this platform has its limitations because, for example, of the absence of eukaryotic posttranslational modifications and improper folding of many complex human proteins. During the last three decades many research laboratories have focused their interests on developing alternative systems for expressing recombinant proteins that could overcome the shortcomings of bacterial systems. Emerging out of these studies were animal and insect cell culture systems. Although a number of products such as monoclonal antibodies, vaccines and therapeutics have been produced using these systems, but the high cost of production combined with the requirement of highly sophisticated manufacturing facilities for each target protein motivated the search for different production systems.
In recent years, plants have been increasingly used as a host system for the expression of recombinant proteins. Such expression can be accomplished, for example, either by integrating the gene of interest into a plant genome, to create a transgenic plant that stably expresses the desired protein, or by introducing the gene of interest into a plant vector that can be introduced into, and transiently maintained in, plant cells. Viral vector systems have proven to be particularly useful.
However, there remains a need for developing improved systems for expression of a molecule of interest in plants. For example, viruses may infect non-target plants, potentially posing significant environmental risks. Also, many available engineered plant viruses do not express inserted genes at desired levels, and/or in desired target plants or tissues. In addition, one disadvantage with various existing viral vector systems is that virus stability can be problematic. In general, there is a need in the art for plant expression systems that would allow for greater flexibility and control.